There is a need for a non-invasive bedside monitor for adult human patients to continuously measure brain oxygenation. Near-infrared spectroscopy (NIRS) is an optically based technique that could meet such demand. However, NIRS instruments have yet to achieve their initially expected potential of becoming routine clinical monitors complementary to pulse oximetry. One major challenge is to develop an optical probe and a corresponding algorithm to solve the problem of interference from extracerebral tissue (skin, scalp, and skull) in adult NIRS applications. Another critical issue is to develop an algorithm that will be able to absolutely quantify the optical signals to derive quantitative variables or indices that are of clinical significance. Our goal is to resolve these issues by developing an NIRS monitor that will determine absolute brain hemoglobin oxygen saturation on adult patients. In Phase I, we have determined an optimum adult NIRS probe configuration to minimize extracerebral interference. We quantitatively demonstrated that the effect of extracerebral tissue oxygenation interference is reduced using a novel algorithm with experiments using the pig model, and with a preliminary human adult study. We demonstrated that our novel algorithm could determine absolute brain oxygen saturation (SnO2) with a high degree of accuracy over a full range of oxygen saturation in the pig model. The primary objectives of this SBIR Phase II proposal are: 1) To refine the design of a NIRS monitor for use on adult human patients. To construct and test improved prototypes and to determine pre-manufacturing requirements of a commercial adult NIRS monitor system. 2) To validate our NIRS monitor for measuring absolute oxygen saturation in healthy human adults. [unreadable] [unreadable]